For example, JP-2008-39614A discloses a sensor for detecting acceleration and angular velocity. The sensor includes a vibrator configured to be displaced in both a drive direction and a detection direction perpendicular to the drive direction, a driver for causing the vibrator to vibrate in the drive direction, and detection electrodes for detecting a displacement of the vibrator in the detection direction based on a change in its capacitance.
The sensor disclosed in JP-2008-39614A detects acceleration and angular velocity by detecting the capacitance change of the detection electrodes. That is, both acceleration and angular velocity are detected based on the capacitance change of the detection electrodes. Therefore, it is difficult to determine a structural resonance, which is in proportion to the square root of a spring constant and in inverse proportion to the square root of the mass, suitable for both acceleration and angular velocity detection.
The detection electrodes are arranged in parallel to the drive direction and coupled together by a detection spring having elasticity in the detection direction. Further, the detection electrodes are coupled together in the drive direction by a coupling link spring.
JP-2008-39614A describes as follows: “When acceleration is applied, the detection spring is displaced in the same phase in the detection direction. Therefore, resonance frequency (structural resonance) at the time of detection of acceleration depends on the detection spring. In contrast, when angular velocity is applied, the detection spring is displaced in opposite phases in the detection direction. Therefore, the resonance frequency at the time of detection of angular velocity depends on both the detection spring but also the coupling link spring. For this reason, the resonance frequency at the time of detection of acceleration and the resonance frequency at the time of detection of angular velocity can be separated from each other by adjusting a spring constant of the coupling link spring”.
However, unless the detection electrodes are ideally displaced in the same phase in the detection direction, the resonance frequency at the time of detection of acceleration may depend on not only the detection spring but also the coupling link spring. In practice, the detection electrodes cannot be ideally displaced in the same phase in the detection direction due to factors such as manufacturing variations. If the detection electrodes are not ideally displaced in the same phase in the detection direction, the detection electrodes are obliquely positioned with respect to each other. As a result, since biasing force of the coupling link spring acts on the detection electrodes in the detection direction, the coupling link spring contributes to the structural resonance at the time of detection of acceleration. Therefore, the structural resonance at the time of detection of acceleration substantially depends on both the detection spring and the coupling link spring.
As described above, according to the sensor disclosed in JP-2008-39614A, each of the resonance frequency at the time of detection of acceleration and the resonance frequency at the time of detection of angular velocity depends on both the detection spring and the coupling link spring. Therefore, it is difficult to determine the structural resonance suitable for both acceleration and angular velocity detection.
For example, when the structural resonance (i.e., spring constant) is reduced, it becomes likely that the detection electrodes are displaced by an inertial force. As a result, the amount of the capacitance change of the detection electrodes due to the inertial force is increased so that a sensitivity characteristic of the acceleration sensor can be improved. However, in this case, since it becomes likely that the detection electrodes are displaced by an external force, an impact resistance of the angular velocity sensor may be reduced.
Conversely, when the structural resonance (i.e., the spring constant) is increased, it becomes less likely that the detection electrodes are displaced by the inertial force. As a result, it becomes less likely that the detection electrodes are displaced by the external force so that the impact resistance of the angular velocity sensor can be improved. However, in this case, since it becomes less likely that the detection electrodes are displaced by the inertial force, the sensitivity characteristic of the acceleration sensor may be reduced.